MSY

Last updated: 12/08/2014

Maximum Sustainable Yield (MSY) is the theoretical largest yield, or catch, that can be taken from a stock over an indefinite period, while still maintaining population size at the point of maximum growth. This by harvesting the individuals that would normally be added to the population.

In other words, it can be defined as the maximum killing of individuals in a population that can occur, while still maintaining a viable population size.

MSY is built on the assumption that reproduction increases with decrease in population density, to a certain point where reproduction will reach its maximum. Under the assumption of logistical growth, this point will be at half the carrying capacity of a species, in other words at half the unfished size. However, in practical fisheries management this level varies depending on the species and its populations’ characteristics and the nature of the fishery exploiting the species.

We are of the opinion that under current circumstances MSY exploitation constitutes a major improvement in fisheries management for a majority of the European fish stocks. However, we believe that in a longer term perspective more ambitious harvest control rules beyond MSY are needed. Therefore, as stated in the UN Fish Stocks Agreement, MSY should only be considered an intermediate target to achieving abundance. Alternative objectives must be developed that are more conservative and precautionary in nature.

For decades scientists have strongly challenged the concept of MSY as a valid management objective from an economical as well as biological point of view. MSY as a simple calculation ignores the size and age of the individuals being taken, their reproductive capacity, assumes a constant recruitment level, and it is single species oriented, not taking into account wider ecosystem effects such as predator-prey interactions. Furthermore, MSY is a maximum value beyond which productivity is assumed to decline. It is calculated using estimates rather than sound data, meaning it can easily lead to the over-exploitation of fish stocks.

Instead we would like to see more ambitious harvest strategies in the future that are more precautionary in nature and more compatible with ecosystem based management, such as the generic harvest control rules proposed by Froeser et al in 2010.